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Question
Find the sum of first seven numbers which are multiples of 2 as well as of 9.
Solution
For finding the sum of first seven numbers which are multiples of 2 as well as of 9.
Take LCM of 2 and 9 which is 18.
So, the series becomes 18, 36, 54,...
Here, first term (a) = 18,
Common difference (d) = 36 – 18 = 18
∵ Sn = `n/2[2a + (n - 1)d]`
S7 = `7/2[2(18) + (7 - 1)8]`
= `7/2[36 + (6 xx 18)]`
= 7(18 + 3 × 18)
= 7(18 + 54)
= 7 × 72
= 504
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